Over the last few decades, the electronics industry has undergone a revolution by the use of semiconductor technology to fabricate small, highly integrated electronic devices. The most common semiconductor technology presently used is silicon-based. A large variety of semiconductor devices have been manufactured having various applicabilities and numerous disciplines. An example of such a silicon-based semiconductor device is a metal-oxide-semiconductor (MOS) transistor.
The principal elements of a typical MOS semiconductor device are illustrated in FIG. 1. The device generally includes a gate electrode 101, which acts as a conductor, to which an input signal typically is applied via gate terminal (not shown). Heavily doped source region 103 and drain region 105 are formed in a semiconductor substrate 107, and respectively are connected to source and drain terminals (not shown).
A channel region 109 is formed in the semiconductor substrate 107 beneath the gate electrode 101 and separates the source region 103 and drain region 105. The channel typically is lightly doped with a dopant of a type opposite to that of the source and drain regions. The gate electrode 101 is physically separated from the semiconductor substrate 107 by a gate insulating layer 111. Typically, this insulating layer is an oxide layer such as SiO.sub.2. The insulating layer 111 is provided to prevent current from flowing between the gate electrode 101 and the semiconductor source region 103, drain region 105, or channel region 109.
In operation, an output voltage typically is developed between the source and drain terminals. When an input voltage is applied to the gate electrode 101, a transverse electric field is set up in the channel region 109. By varying the transverse electric field, it is possible to modulate the conductance of the channel region 109 between the source region 103 and drain region 105. In this manner, an electric field controls the current flow through the channel region 109. This type of device commonly is referred to as a MOS field-effect-transistor (MOSFET). Semiconductor devices such as the one described are used in large numbers to construct most modern electronic devices. In order to increase the capability of such electronic devices, it is necessary to integrate ever larger numbers of such devices into a single silicon wafer. As the semiconductor devices are scaled down in order to form a larger number of such devices on a given surface area, the structure of the devices and fabrication techniques used to make the devices must be altered.
One important step in the manufacture of MOS devices is the fabrication of the gate electrode. The gate electrode typically is formed by depositing a layer of polysilicon. Portions of the polysilicon layer are removed selectively, for example, using well-known photolithography and etching techniques. However, these conventional techniques for forming gate electrodes impose limitations on the minimum width of the gate electrode. The resolution of the photolithography process, in particular, imposes limitations on the minimum width of the gate electrode. As these thresholds for minimum width are reached, the ability to further scale down the semiconductor devices is hindered.